The present invention relates to a method and apparatus for previewing or screening photographic films or the like, especially to a method and apparatus for examining and evaluating successive originals, such as exposed and developed photographic film frames, prior to introduction into a copying machine. More particularly, the invention relates to improvements in a method and apparatus for examining exposed and developed film frames in order to detect those frames whose quality is unsatisfactory for the making of reproductions, such as prints on photographic paper or another suitable carrier material.
As a rule, exposed customer films which have been delivered, shipped or mailed to a processing laboratory are spliced together end-to-end to form an elongated web which is thereupon caused to advance through a developing machine prior to introduction into a copying apparatus. Developed films are examined prior to introduction into the copier in order to detect and identify those film frames whose quality is unsatisfactory for the making of acceptable prints. The examination of film frames is often automatic, i.e., the equipment which is associated with or incorporated in a copier is sufficiently sophisticated to detect those frames whose images are underexposed or overexposed to such an extent that they are evidently unfit for the making of acceptable prints. However, though highly or extremely complex and expensive, such automatic equipment cannot discriminate between satisfactory film frames and those frames whose images are blurred due to camera shake or improper focussing. At the present time, such defective frames must be detected by a skilled person who examines successive frames prior to their introduction into the copying machine. Detection of film frames which are defective but cannot be identified by presently known automatic equipment is desirable because prints of images of such frames are a frequent cause of dissatisfaction, i.e., the customers are not willing or are reluctant to pay for prints of film frames bearing images which are not worthy of reproduction.
German Offenlegungsschrift No. 2,244,340 discloses a procedure for automatic screening of exposed and developed photographic film frames for the purpose of detecting that frame or those frames which cannot be used for the making of satisfactory prints. In accordance with the proposal in this publication, the image on a film frame is monitored by photoelectric means and the intensities of signals are obtained in response to monitoring of neighboring fields are compared with threshold values. The number and/or intensities of comparison signals whose intensity exceeds the respective threshold value are totalized, and the thus obtained signal is compared with a reference value which is indicative of acceptable sharpness of an image. If the intensity or another characteristic of the signal representing the sum total of aforementioned comparison signals does not match the corresponding characteristic of the reference signal, the image of the film frame is not reproduced during travel through the copying machine. A drawback of the just discussed proposal is that the apparatus is incapable of adequately discriminating between sharp and blurred images, especially when the contrast is low and the number of details in the scanned image is small.
The German-language publication "Bild und Ton" (Vol. 8, 1970, pp. 229-236) discloses a different proposal to ascertain the sharpness of images on film frames. This publication suggests to limit the density contrast measurement to that field or region of a frame which is critical for the determination of sharpness. Such proposal, too, is not entirely satisfactory because the measurement of density contrast and/or density gradient is overly dependent of the motif and contrast of the scanned image. The number of errors is much too high to warrant resort to such a technique for automatic determination of film frames which are unfit for reproduction.